Ceramic composites are becoming of increasing importance as construction materials for components such as gas turbine parts, air frame components, rocket engine components, pump impellers and the like which are exposed to high temperature environments and/or abrasive or corrosive conditions. Often ceramic materials are reinforced to increase resistance to fracture by the addition of a dispersed second phase or fibers such as with the ceramic composites of Al.sub.2 O.sub.3 --ZrO.sub.2, Si.sub.3 N.sub.4 --SiC, Al.sub.2 O.sub.3 --TiC, fiber reinforced silicon carbide, and partially stabilized ZrO.sub.2. The best known composites are made from two-dimensional fabrics and/or fibers dispersed in a resin or plastic matrix. These composites are basically a resin or plastic structure to which reinforcing fabrics or fibers have been added to enhance the physical properties of the structure.
A wide range of multidirectional reinforced composite structures are now available. The simplest of these structures is obtained by stacking unidirectional fibers or sheets with alternating layers oriented in difference directions, or by stacking woven sheets. More complex structures provide three-dimensional reinforcement. The simplest of the three-dimensional structures is the three-directional (3D) structure which generally has reinforcing elements which are mutually orthogonal. The most complex three-dimensional structure is the thirteen-directional (13D) structure. The thirteen directions, with reference to a cube, form three subgroups; the three edges, the four long diagonals, and the six diagonals of the faces.
Carbon-carbon composites are a class of materials whose properties, especially at elevated temperatures, make them attractive for various aerospace applications. The materials are composites although all of the composite elements are comprised essentially of carbon, in its various allotropic forms. Carbon-carbon materials are produced starting with organic precursor fibers such as polyacrylonitrile, rayon or pitch. Such fibers are usually produced in bundles (yarn), often by an extrusion process. The precursor fibers are heated in an inert atmosphere to pyrolyze or carbonize them and may then be heated to a higher temperature (e.g. 2204.degree. C.) to form graphite fibers. These carbon or graphite materials may then be laid down, woven, or interleaved to form what are referred to as 1D, 2D, 3D, etc. structures where D stands for direction (i.e. in a 2D structure fibers are laid in two, usually orthogonal, directions). These woven structures can then be impregnated with a pitch or resin material which is converted to carbon and then graphite. In this process, hot pressing is sometimes employed to obtain a dense structure. Repeated impregnation steps can be employed to increase density. An alternative processing scheme is to use chemical vapor deposition (CVD) to deposit pyrolytic graphite on the woven structures to densify the structure.
The drawback to these ceramic composites is their susceptibility to oxidative and corrosive attacks, particularly at high temperatures. Coatings have been applied to protect the composites from oxidation and corrosion. However, major difficulties have been encountered with coated ceramic materials as shown in FIG. 3. Proper adhesion of the coating to the ceramic can be difficult because of the stresses which develop due to the varying degrees of thermal expansion of the ceramic and the coating. As a result, especially in high temperature applications, cracking of the coating frequently occurs allowing oxidation and a corrosive attack on the ceramic substrate. In addition, mechanical vibrations and other forms of physical stress or even debris damage may cause cracking and spalling of the protective coating layer if proper adhesion of the coating to the ceramic is not achieved.
Various attempts have been made in the art to relieve the foregoing problems associated with thermal expansion mismatch. Among the solutions includes coating the ceramic with an oxygen scavenging sealant layer to provide protection or a gradient in the thermal coefficient of expansion from the ceramic substrate to the outer oxidation resistant coating. A carbon body with an oxidation resistant coating is disclosed in U.S. Pat. No. 4,515,860 in which a coating is formed of a silicon alloy having a non-columnar grain distribution.